1. Field of the Invention
The present invention relates to a process for preparing potassium ferrate (K.sub.2 FeO.sub.4) from potassium hydroxide, chlorine and ferric salts. In particular, the present invention relates to an economically-feasible process for making potassium ferrate in large-scale quantities.
2. Description of the Prior Art
Potassium ferrate has many known uses. For example, it may be employed to bleach vegatable fibers; to effect organic reactions, to oxidize sulfurous acid, nitrites, ferrocyanides and other inorganic materials (see U.S. Pat. No. 2,758,090, which issued to Mills et al. on Aug. 7, 1956). It may be also used in the paper industry to make oxidized starches for surface sizing or in the textile industry to make oxidized starches for finishing operations (see U.S. Pat. No. 3,632,802 which issued to DeMiller et al. on Jan. 4, 1972). It may be employed to decolorize and remove impurities from alkali metal hydroxide solutions (see U.S. Pat. No. 2,536,703, which issued to Schreyer on Jan. 2, 1951).
Furthermore, potassium ferrate may be useful in oxidizing H.sub.2 S in gas streams directly to sulfur (see Japanese Patent Kokai No. 74/032896 which was published on Apr. 8, 1974 and assigned to Kobayashi). It may also be used to remove manganese, antimony and arsenic during the production of zinc (see U.S.S.R. Invention Certificate No. 378,472, which issued to Nerezov et al. in April, 1973). Also, it may be used in the treatment and purification of water [see Murmann et al., "Experiments Utilizing FeO.sub.4.sup.-2 for Purifying Water", Water Research, Vol. 8, pages 79 to 83 (1974); Gilbert et al., "An Investigation of the Applicability of Ferrate Ion for Disinfection", J. Am. Water Works, Vol. 68(9), pages 495-497 (1976) and Waite et al. "Iron (VI) Ferrate As a General Oxidant For Water and Wastewater Treatment", Ozone/Chloride Dioxide Oxid. Prod., Org. Matter Process Conference, (1976)].
Still further, potassium ferrate may be used in non-ferrous metallurgy and mining operations (see U.S.S.R. Invention Certificate No. 432,765, which issued to Nerezov et al. on Oct. 27, 1977 and U.S.S.R. Invention Certificate No. 639,606, which issued to Konev et al on Dec. 3, 1978). In addition, potassium ferrate may be used in cigarette filters (see U.S. Pat. No. 4,246,910, which issued to Rainer et al on Jan. 27, 1981).
Many methods have been suggested for making potassium ferrate [see Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, pages 929-937, Longmans, Green & Co., London (1952)]. One method is by the electrolysis of iron-containing materials in an electrolytic cell containing KOH or the like [see Grube et al., "The Effect of Superimposed Alternating Current On Anodic Ferrate Formation", Zeitschrift Fur Elektrochemie, Vol. 26, No. 7/8, pages 153 to 161 (1920) and Tousek, "Electrochemical Production of Sodium Ferrate", Collection Czechoslov. Chem. Commun., Vol. 27, pages 914 et seq. (1962)].
Another method for making potassium ferrate is by fusing iron or ferric oxide (Fe.sub.2 O.sub.3) with potassium nitrate in the presence of KOH (see Japanese Patent Kokai No. 80/75926, which was published on June 7, 1980 to T. Morishita).
Still another approach is the reaction of an alkali metal hypohalite salt with ferric salts or the like. Two techniques for making K.sub.2 FeO.sub.4 by this hypohalite/ferric salt reaction have been suggested. One method is to first prepare Na.sub.2 FeO.sub.4 in a NaOH solution by reacting a ferric salt or the like with a sodium hypohalite (e.g. NaOCl) or halogen gas (e.g. Cl.sub.2) in the presence of concentrated NaOH and then converting the Na.sub.2 FeO.sub.4 into K.sub.2 FeO.sub.4 by addition of KOH [see Hrostowski et al., "The Magnetic Susceptibility of Potassium Ferrate", Journal of Chemical Physics, Vol. 18, No. 1, pages 105-107 (January 1950); Thompson et al, "Preparation and Purification of Potassium Ferrate(VI)", J. Am. Chem. Soc., Vol. 73, pages 1379-1381 (March 1951); Schreyer et al., "Potassium Ferrate", Inorganic Synthesis, Vol. IV, pages 164-169 (1953); and Morishita, "Production Method For Alkali Ferrates", Japanese Patent Kokai No. 80/75926, published on June 7, 1980]. While Na.sub.2 FeO.sub.4 is highly soluble even in concentrated NaOH solutions, K.sub.2 FeO.sub.4 will immediately precipitate from concentrated alkali metal hydroxide solutions.
The second hypohalite/ferric salt reaction technique is to directly react potassium hypochlorite, potassium hypobromite, or a halogen gas like Cl.sub.2 or Br.sub.2, with ferric salt or freshly made Fe(OH).sub.3 in the presence of concentrated KOH to form and precipitate K.sub.2 FeO.sub.4 [see U.S. Pat. No. 2,455,696, which issued to Mosesman on Dec. 7, 1948; Helferich et al., "Salts of Ferric Acids", Z. anorg. allg. Chemie, 263, pages 169-174 (1950); Scholder et al., "Concerning Ferrates(VI)", Z. anorg. allg. Chemie, 282, pages 268-279 (1955); Audette et al., "Potassium, Rubidium, Cesium and Barium Ferrate(VI): Preparation, Infrared Spectra and Magnetic Susceptibilities," Inorganic Chemistry, Vol. 11, No. 8, pages 1904-1908 (1972); and Morishita, "Production Method for Alkali Ferrates", Japanese Kokai No. 80/75926, published on June 7, 1980].
Potassium ferrate is a strong oxidizing agent and it readily degrades or reacts in many ways. For example, it quickly degrades in aqueous solutions to produce ferric hydroxide according to the reaction of following Equation (A): EQU 4K.sub.2 FeO.sub.4 +10H.sub.2 O.fwdarw.4Fe(OH).sub.3 +8KOH+30.sub.2(A)
While the rate of this decomposition becomes lower with an increase in alkalinity, the Fe(OH).sub.3 produced has a catalytic effect on this decomposition reaction. Thus, the decomposition of the ferrate ion (FeO.sub.4.sup.-2) is greatly promoted in the presence of certain amounts of ferric hydroxide. However, this Fe(OH).sub.3 produced also has useful functions, including acting as an absorbent.
It is also known that the presence of very small amounts of certain metal impurities (e.g. nickel and cobalt) and organic impurities causes the degradation or decomposition of ferrate(VI) ions, even in strongly alkaline solutions [see Ettel et al., "Reactions of Very Pure Substances (V)--Ferrate Decomposition in Alkaline Solution", Collection Czechoslov. Chem. Commun., Vol. 34, pages 2182 to 2188 (1969)].
Still further, it is known that a large excess of reactants (e.g. Cl.sub.2) or co-products of ferrate formation [e.g. H.sub.2 O, KCl, Fe(OH).sub.3 ] cause an acceleration of the decomposition of ferrate(VI) ions in solution [see Schreyer et al., "Stability of Ferrate(VI) Ion in Aqueous Solution", Anal. Chem., 23, No. 9, pages 1312, 1314 (September 1951)]. Thus, it can be strongly suspected that such degradation problems, among others, have prevented the commercial production of potassium ferrate.
Among the above-noted four general methods for making potassium ferrate, it appears that the second hypohalite/ferric salt technique (where K.sub.2 FeO.sub.4 is made in one step from KOH) is economically and practically advantageous over the other three methods. The direct electrolysis method is impractical for continuous operation because of passivation of the iron anode, which causes an increase in voltage and a decrease in ferrate production. Also, the iron anide is sacrificed and would need to be replaced (resulting in process shutdown). Furthermore, the yields of this technique are small. The iron/potassium nitrate fusing method requires high temperatures (and much energy to reach those temperatures) to accomplish its desired result. The yields of this reaction are also small. The two-state hypohalite/ferric salt reaction with a Na.sub.2 FeO.sub.4 intermediate has commercial disadvantages, namely, it requires extra steps; it forms undesirable sodium impurities as a by-product in the first stage; and it results in a mother liquor comprising a mixture of NaOH and KOH solutions, whose reuse is difficult.
Because potassium ferrate appears to be relatively environmentally safe and offers great potential as an oxidizer in a variety of applications, there is a great need to find an improved process which makes K.sub.2 FeO.sub.4 readily and economically. It is believed that the present invention is a solution to that need.